Multiplexed fiber-optic Fabry-Pérot cavities for refractive index and temperature sensing fabricated using diamond-blade dicing
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2021-01-01 |
| Journal | EPJ Web of Conferences |
| Authors | Ivonne Pfalzgraf, Sergiy Suntsov, Kore Hasse, Detlef Kip |
| Institutions | Helmut Schmidt University |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”This research demonstrates a highly compact, multiplexed fiber-optic sensor array fabricated using precision diamond-blade dicing, achieving simultaneous, spatially resolved refractive index (RI) and temperature (T) sensing.
- Core Technology: Up to four open Fabry-Pérot (FP) micro-cavities are diced into a single-mode optical fiber tip, creating an ultra-compact “lab-on-a-fiber” device.
- Performance Enhancement: Reflectivity is significantly increased using evaporated single layers of high-index Tantalum Pentoxide (Ta2O5) coatings on the cavity sidewalls.
- Multiplexed Sensing: The sensor simultaneously measures RI distributions across four distinct spatial locations.
- Temperature Compensation: Real-time temperature cross-talk is eliminated by using adjacent solid fiber core sections or a high thermo-optic coefficient Silicon (Si) inlay as dedicated temperature sensors.
- High Accuracy: After temperature calibration, the sensor achieved a corrected RI measurement accuracy as high as 6x10-6 RIU.
- Sensitivity: Average RI sensitivity across the four sensors was approximately 1185 nm/RIU, with temperature sensitivity up to 79 pm/°C (using the Si inlay).
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| Maximum Multiplexing | 4 | Cavities | Open FP RI sensors read simultaneously. |
| RI Sensitivity (Average) | ~1185 | nm/RIU | Achieved for coated open cavities. |
| RI Accuracy (Corrected) | 6x10-6 | RIU | Deviation of corrected RI change (Δnm). |
| RI Accuracy (Si Inlay Sensor) | 2x10-5 | RIU | Spatially resolved, temperature-compensated sensing. |
| Temperature Sensitivity (Si Inlay) | 79 | pm/°C | Highest sensitivity achieved due to Si thermo-optic coefficient (1.7x10-4 K-1). |
| Temperature Sensitivity (Solid Fiber) | 19.4 | pm/°C | Typical sensitivity for solid core sections (FPR5). |
| Temperature Accuracy | 0.01 | °C | Achieved using solid fiber sections. |
| Ta2O5 Refractive Index | 2.07 | (unitless) | Measured at 1550 nm. |
| Interrogation Wavelength Range | 1460 - 1620 | nm | Micron Optics Hyperion Si155 system. |
| Interrogation Accuracy | 1 | pm | Wavelength measurement accuracy. |
| Water Bath Accuracy | 0.01 | °C | Temperature stabilization accuracy (Julabo FP45). |
Key Methodologies
Section titled “Key Methodologies”The FP micro-cavities were fabricated using precision diamond-blade dicing, followed by thin-film deposition for enhanced performance.
- Fiber Preparation: The single-mode optical fiber was secured inside a quartz ferrule to maximize mechanical stability during the dicing process.
- Cavity Dicing: A Disco DAD322 wafer saw was used to cut slots through the fiber core, creating open FP micro-cavities. Resin-based blades with very fine grit were employed to ensure smooth cavity facets.
- Cut Geometry Control: Blade conditioning using a special dresser board ensured that all cuts maintained a nearly rectangular shape.
- Reflectivity Coating: Single layers of high-index Ta2O5 (n = 2.07 at 1550 nm) were evaporated onto the sidewalls of the open cavities to increase reflectance.
- Coating Optimization: Coating thicknesses were precisely controlled during evaporation while monitoring the reflection spectra. For example, the coating for FPR4 was a quarter-wave layer, while the coating for FPR3 was significantly thinner (1/6.5 of the FPR4 thickness) to optimize the strength of the corresponding FFT peaks.
- Si Inlay Integration (Optional): A rectangular Si plate was prepared using the dicing saw, placed inside a designated open cavity, and secured using NOA61 adhesive to serve as a high-sensitivity temperature reference element.
- Data Acquisition: An optical interrogation system (Micron Optics Hyperion Si155) was used to record reflection spectra (1460-1620 nm).
- Sensing Extraction: Phase tracking of the essential fast Fourier transform (FFT) components was used to extract small changes in the optical length (Lo) induced by RI or temperature changes.
Commercial Applications
Section titled “Commercial Applications”This multiplexed fiber sensor technology is highly relevant for applications requiring compact, high-accuracy, and spatially resolved measurements in liquid media.
- Biomedical Diagnostics:
- Monitoring RI and temperature changes in extremely small volume samples (e.g., microfluidic chips, single-cell analysis).
- Real-time monitoring of biochemical reactions where RI changes correlate with reaction progress or concentration.
- Lab-on-a-Fiber Systems:
- Creating ultra-compact chemical sensors by filling or coating the FP cavities with functional layers (e.g., molecularly imprinted polymers) sensitive to specific trace substances.
- Industrial Process Control:
- High-precision, spatially resolved monitoring of liquid concentrations (e.g., sucrose, solvents) in pharmaceutical or food processing pipelines.
- Quality control requiring simultaneous measurement of concentration (RI) and thermal stability (T).
- Environmental Sensing:
- Remote sensing applications where immunity to electromagnetic interference and small sensor size are critical for deployment in harsh or complex environments.
View Original Abstract
We report on multiplexing several Fabry-Pérot (FP) cavities in single-mode optical fibers for highprecision spatially resolved sensing of refractive indices (RI) of liquids. Resonators are fabricated by cutting small slots into fibers using a diamond-blade dicing saw and additional coating with thin Ta 2 O 5 layers to increase cavity reflectance. Temperature compensation of RI measurements is achieved either by evaluating the reflection signals resulting from the solid core parts between different open-cavity sensor elements, or by using a thin Si inlay glued into one of the open cavities. The multiplexing performance and accuracy of the fabricated sensors with up to four open cavities were tested on sucrose solutions over a range of temperatures.